clang-format: Introduce BreakConstructorInitializers option

Summary:
This option replaces the BreakConstructorInitializersBeforeComma option with an enum, thus introducing a mode where the colon stays on the same line as constructor declaration:

  // When it fits on line:
  Constructor() : initializer1(), initializer2() {}

  // When it does not fit:
  Constructor() :
      initializer1(), initializer2()
  {}

  // When ConstructorInitializerAllOnOneLineOrOnePerLine = true:
  Constructor() :
      initializer1(),
      initializer2()
  {}

Reviewers: krasimir, djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D32479

llvm-svn: 303739